This invention relates to catalytic cracking and, more particularly, to a process for increasing the yield in a catalytic cracking unit.
Catalytic cracking of oil is an important refinery process which is used to produce gasoline and other hydrocarbons. During catalytic cracking, the feedstock, which is generally a cut or fraction of crude oil, is cracked in a reactor under catalytic cracking temperatures and pressures in the presence of a catalyst to produce more valuable, lower molecular weight hydrocarbons. Gas oil is usually used as a feedstock in catalytic cracking. Gas oil feedstocks typically contain from 55% to 80% gas oil by volume having a boiling range from 650.degree. F. to 1000.degree. F. and less than 1% RAMS carbon by weight. Gas oil feedstocks also typically contain less than 5% by volume naphtha and lighter hydrocarbons having a boiling temperature below 430.degree. F., from 10% to 30% by volume diesel and kerosene having a boiling range from 430.degree. F. to 650.degree. F., and less than 10% by volume resid having a boiling temperature above 1000.degree. F. It is desirable to provide an effective process to increase the yield of gasoline (naphtha) in catalytic cracking units.
It has been known to deasphalt and catalytically crack virgin unhydrotreated, low sulfur resid as well as to deasphalt, subsequently hydrotreat, and catalytically crack high sulfur resid. Better rates and extent of resid conversion are desirable, however. Furthermore, such prior art processes produce hydrogen-rich asphaltenes that are difficult and expensive to handle and process, melt (liquefy) at relatively low temperatures and cannot be used as solid fuel, are difficult to blend into fuel oils, and are not generally usable and desirable for asphalt paving or for use in other products.
In the past, spiraling oil costs and extensive price fluctuations have created instability and uncertainty for net oil consuming countries, such as the United States, to attain adequate supplies of high-quality, low-sulfur, petroleum crude oil (sweet crude) from Nigeria, Norway, and other countries at reasonable prices for conversion into gasoline, fuel oil, and petrochemical feedstocks. In an effort to stabilize the supply and availability of crude oil at reasonable prices, Amoco Oil Company has developed, constructed, and commercialized extensive, multimillion dollar refinery projects under the Second Crude Replacement Program (CRP II) to process poorer quality, high-sulfur, petroleum crude oil (sour crude) and demetalate, desulfurize, and hydrocrack resid to produce high-value products, such as gasoline, distillates, catalytic cracker feed, metallurgical coke, and petrochemical feedstocks. The Crude Replacement Program is of great benefit to the oil-consuming nations by providing for the availability of adequate supplies of gasoline and other petroleum products at reasonable prices while protecting the downstream operations of refining companies.
During resid hydrotreating, such as under Amoco Oil Company's Crude Replacement Program, resid oil is upgraded with hydrogen and a hydrotreating catalyst to produce more valuable lower-boiling liquid products. Undesirably, carbonaceous solids are formed, however, during resid hydrotreating. These solids have been characterized as multicondensed aromatics which form and precipitate from cracking of the side chains of asphaltenes. The solids are substantially insoluble in hexane, pentane, and in the effluent hydrotreated product oil. The solids become entrained and are carried away with the product. Such solids tend to stick together, adhere to the sides of vessels, grow bigger, and agglomerate. Such solids are more polar and less soluble than the residual oil feedstock.
Carbonaceous solids are produced as a reaction byproduct during ebullated bed hydrotreating (expanded bed hydrotreating). During ebullated bed hydrotreating, the ebullating hydrotreating catalyst fines serve as a nucleus and center for asphaltene growth. The situation becomes even more aggravated when two or more hydrotreating reactors are connected in series as in many commercial operations. In such cases, solids formed in the first reactor not only form nucleation sites for solids growth and agglomeration in the first reactor, but are carried over with the hydrotreated product oil into the second reactor, etc., for even larger solids growth and agglomeration.
The concentration of carbonaceous solids increases at more severe hydrotreating conditions, at higher temperatures and at higher resid conversion levels. The amount of carbonaceous solids is dependent on the type of feed. Resid conversion is limited by the formation of carbonaceous solids.
Solids formed during resid hydrotreating cause deposition and poor flow patterns in the reactors, as well as fouling, plugging, and blocking of conduits and downstream equipment. Oils laden with solids cannot be efficiently or readily pipelined. Hydrotreating solids can foul valves and other equipment, and can build up insulative layers on heat exchange surfaces reducing their efficiency. Buildup of hydrotreated solids can lead to equipment repair, shutdown, extended downtime, reduced process yield, decreased efficiency, and undesired coke formation.
Generally, organometallic compounds are substantially heavier than the oils and are associated with the asphaltenes in the heavy hydrocarbon materials. However, some of the organometallic compounds are associated with the resins and some of the heavier oils in the heavy hydrocarbon materials. The presence of organometallic compounds in the separated oils fraction is undesirable. The metals tend to poison catalysts employed in refining processes to upgrade the oils fraction into other useful products.
Over the years, a variety of processes and equipment have been suggested for various refining operations, such as for upgrading oil, hydrotreating, reducing hydrotreated solids, and catalytic cracking. Typifying some of these prior art processes and equipment are those described in U.S. Pat. Nos.: 2,382,382; 2,398,739; 2,398,759; 2,414,002; 2,425,849; 2,436,927; 2,884,303; 2,981,676; 2,985,584; 3,004,926; 3,039,953; 3,168,459; 3,338,818; 3,351,548; 3,364,136; 3,513,087; 3,563,911; 3,661,800; 3,766,055; 3,838,036; 3,844,973; 3,905,892; 3,909,392; 3,923,636; 4,191,636; 4,239,616; 4,290,880; 4,305,814; 4,331,533; 4,332,674; 4,341,623; 4,341,660; 4,400,264; 4,454,023; 4,486,295; 4,478,705; 4,495,060; 4,502,944; 4,521,295; 4,526,676; 4,592,827; 4,606,809; 4,617,175; 4,618,412; 4,622,210; 4,640,762; 4,655,903; 4,661,265; 4,662,669; 4,692,318; 4,695,370; 4,673,485; 4,681,674; 4,686,028; 4,720,337; 4,743,356; 4,753,721; 4,767,521; 4,769,127; 4,773,986; 4,808,289; and 4,818,371. These prior art processes and equipment have met with varying degrees of success.
It is, therefore, desirable to provide an improved catalytic cracking process for increasing the yield of more valuable liquid products.